1. Field of the Invention
This invention relates to improvements in stacked multicell batteries, and more particularly to providing a conductive medium between electrodes while preventing ionically conductive paths from forming between adjacent cells and the battery case.
2. Description of the Prior Art
When constructing practical electrochemical cells into batteries, there are two basic ways in which electrodes can be connected inside the cell or battery module case. These are series and parallel connections. In a bipolar battery design, the electrodes are hooked together in series, thus the voltage of the stack is n times that of a single cell, where n is equal to the number of cells in the stack. Each cell comprises a positive and a negative electrode separated from one another by a separator. The separator is an insulative material that prevents the anode and cathode from physically touching while allowing for ionic conduction between the electrodes. Adjacent cells are separated by respective bipolar walls. The bipolar walls are plates which allow an electronic path between adjacent cells while not allowing an ionic path.
When the liquid electrolyte contacts the bipolar wall, it may either ball up or it may run across the surface of the bipolar wall. When the liquid electrolyte runs across the surface of the bipolar wall, the wall is said to be wetted by the electrolyte. It is when the bipolar wall is wetted by the electrolyte that the electrolyte might travel in an undesirable path to the adjacent cell. The bipolar wall of a lithium metal sulfide battery must have several characteristics for successful application. It must not be corroded by the electrode or electrolyte materials, it must conduct electricity through its thickness, and it must not be wet by electrolyte at its edges. The nonwetting quality is required to prevent shorting of the adjacent cells by surface tension driven creep of electrolyte around the bipolar plate.
The positive and negative electrodes of each cell contain an electrolyte which is liquid at the operating temperature. The bipolar wall separates adjacent cells and is designed to allow an electronic path between adjacent cells while not allowing an ionic path. If a path of electrolyte is allowed to travel around the bipolar wall, an ionic short develops reducing the effectiveness of the battery. Preventing an ionic short due to an electrolyte travelling around the bipolar wall ensures that all of the current flow through the bipolar wall should be electronic in nature and that there is no ionic flow.
The ideal solution to the electrolyte leakage problem is to use a minimum amount of electrolyte which is completely contained within the individual electrodes and separators by capillary forces. In practice, however, this is almost impossible since more than this minimum amount of electrolyte is required to obtain the desired electrochemical performance from the battery. Thus, means must be developed for addressing the problem of the migration of excess electrolyte.
Heretofore, metals have usually been considered for the bipolar plate because of the electrical conductivity requirement. However, metals typically used in the industry do not meet the corrosion resistance requirement or the nonwetting requirement without additional treatment. The only metal that the industry has found to meet the corrosion resistance requirement is molybdenum but molybdenum is wetted by electrolyte so that edge sealing is a problem. The molybdenum must therefore be coated with some other material to remedy the edge sealing problem. Other metals do not satisfy the corrosion resistance requirement.
Because molybdenum is relatively costly, is relatively heavy so as to add weight to the cell stack, and is easily wetted, it would be preferable to replace molybdenum with some other material as the bipolar wall material. If some other material is to be used, it will have to be coated with an electrically conductive coating which is highly resistant to attack by the electrode materials or the electrolyte. If a perfect molybdenum coating could be put down then it would work. However, this is very difficult to accomplish practically. Furthermore, even tiny imperfections (pin holes) in the coating will rapidly result in corrosion of the underlying metal plate. Depending on the coating material, it is even possible that the presence of the coating material will accelerate an attack of the underlying metal in the vicinity of the pin holes, compared to the rate of attack on an uncoated plate. Any full penetration of the plate will short adjacent cells, rendering them ineffectual.